H. Hovel, in "Novel Materials and Devices for Sunlight Concentrating Systems", IBM J. Res. and Dev., 22, 112, (1978), describes the advantages of "multicolor" solar cells. By using several materials with different energy bandgaps in optical series, the usual energy losses which limit solar cell efficiencies to less than 30% are overcome. Efficiencies approaching or above 40% can be achieved by practice of this invention. Conventional schemes for producing cascade cells involve growing one material upon another, to overcome material mismatch problems.
In cases where monocrystalline semiconductor layers are desired and the lattice parameters of the semiconductors are not close enough to the parameters of the substrate to ensure low defect densities and good electrical properties, a crystal perfection accommodation region as outlined in U.S. patent application Ser. No. 968,887, now U.S. Pat. No. 4,202,704 filed Dec. 13, 1978, can be used to improve the properties of the semiconductor solar cell layers. A crystal perfection accommodation region may be another semiconductor, for example, which reduces the stresses and defects that would otherwise result from the lattice mismatch between the substrate and the semiconductor solar cell layers on both sides of the substrate.
Semiconductors grown upon foreign substrates are generally polycrystalline in nature unless special conditions prevail. As an example, layers of GaAs, GaAlAs, GaAsP, Si, Ge and others are polycrystalline when grown upon quartz, which is a transparent insulating material. Under special conditions in which the substrate is monocrystalline and the lattice parameters of the semiconductor are close to those of the substrate, and where the temperature and other growth conditions are carefully chosen, a monocrystalline epitaxial semiconductor layer can be grown upon a transparent insulating substrate. Polycrystalline layers on transparent insulating substrates can form the basis for low cost, moderately high efficiency tandem solar cells while monocrystalline layers on transparent insulating substrates can form the basis for higher efficiency structures which may be more costly but which can be used in concentrating systems where efficiency is more important than cost (see the Hovel article mentioned above).
The following United States of America Patents are of special background interest concerning the solar cells of this invention:
(1) Abrahamsohn (U.S. Pat. No. 3,376,163) discloses a photosensitive device composed of two photovoltaic cells applied to opposite sides of clear glass substrate.
(2) Jackson (U.S. Pat. No. 2,949,498) teaches the use of a variety of semiconductor materials, such as GaAs, InP, Si and Ge, in multilayer solar energy converters wherein each layer has a different energy gap.
(3) Milnes (U.S. Pat. No. 4,094,704) also shows overlying and underlying solar cells on opposite sides of a transparent substrate, wherein the layers may be such as GaAs, GaAlAs, Si.
(4) Mann (U.S. Pat. No. 3,450,568) pertains to solar cells with wrap-around electrodes.
Thus, tandem superimposed cells have been shown for the prior art practice in U.S. Pat. Nos. 2,949,498; 3,376,163 and 4,094,704 wherein the cells either are separate or are associated with the substrate by layers that attenuate the performance. Wrap-around contacts have for other structures been shown in U.S. Pat. No. 3,450,560.